Re: Remove dependence on integer wrapping

On Sun, Jun 09, 2024 at 09:57:54PM -0400, Tom Lane wrote:
> Nathan Bossart <nathandbossart(at)gmail(dot)com> writes:
>> On Sun, Jun 09, 2024 at 04:59:22PM -0400, Joseph Koshakow wrote:
>>> The following comment was in the code for parsing timestamps:
>>> /* check for just-barely overflow (okay except time-of-day wraps) */
>>> /* caution: we want to allow 1999-12-31 24:00:00 */
>>>
>>> I wasn't able to fully understand it even after staring at it for
>>> a while. Is the comment suggesting that it is ok for the months field,
>>> for example, to wrap around? That doesn't sound right to me I tested
>>> the supplied timestamp, 1999-12-31 24:00:00, and it behaves the same
>>> before and after the patch.
>
>> I haven't stared at this for a while like you, but I am likewise confused
>> at first glance. This dates back to commit 84df54b, and it looks like this
>> comment was present in the first version of the patch in the thread [0]. I
>> CTRL+F'd for any discussion about this but couldn't immediately find
>> anything.
>
> I believe this is a copy-and-paste from 841b4a2d5, which added this:
>
> + *result = (date * INT64CONST(86400000000)) + time;
> + /* check for major overflow */
> + if ((*result - time) / INT64CONST(86400000000) != date)
> + return -1;
> + /* check for just-barely overflow (okay except time-of-day wraps) */
> + if ((*result < 0) ? (date >= 0) : (date < 0))
> + return -1;
>
> I think you could replace the whole thing by using overflow-aware
> multiplication and addition primitives in the result calculation.
> Lines 2-4 basically check for mult overflow and 5-7 for addition
> overflow.

Ah, I see. Joe's patch does that in one place. It's probably worth doing
that in the other places this "just-barefly overflow" comment appears IMHO.

I was still confused by the comment about 1999, but I tracked it down to
commit 542eeba [0]. IIUC it literally means that we need special handling
for that date because POSTGRES_EPOCH_JDATE is 2000-01-01.

[0] https://postgr.es/m/CABUevEx5zUO%3DKRUg06a9qnQ_e9EvTKscL6HxAM_L3xj71R7AQw%40mail.gmail.com

--
nathan